A construction machine such as a hydraulic shovel is constructed by including a traveling structure capable of travelling by a traveling motor, a revolving superstructure that is disposed on the traveling structure, and a working device that is mounted to the revolving superstructure to perform an excavating work of earth and sand, or the like. The revolving superstructure has an operator's cab thereon located on one side in a width direction with a mounting section for the working device between both sides. Such a construction machine further has a hydraulic pump that is located on a rear side of the operator's cab to supply pressure oil to the traveling motor and the working device and an engine for operating the hydraulic pump. A typical engine to be mounted on a construction machine is a diesel engine.
Incidentally, in recent years, a urea SCR system has been known as the system which reduces NOx (nitrogen oxide) in the exhaust gas exhausted from a diesel engine, and purifies the exhaust gas, and the urea SCR system is being also applied to a diesel engine mounted on a construction machine.
A urea SCR system is a NOx selective catalytic reduction system that selectively reduces NOx in exhaust gas by using urea water as a reducing agent, and has a urea water injection valve that injects urea water to an exhaust upstream side of a NOx selective catalytic reduction apparatus, a urea water tank for storing the urea water, and a urea water supply device including a supply pump that is interposed in urea water supply piping that connects the urea water tank and the urea water injection valve to feed the urea water under pressure to the urea water injection valve.
In a cold district or the like, cooling water piping in which the cooling water for an engine is circulated is laid along urea water supply piping to be able to warm urea water with the heat of the cooling water for the engine, in order to prevent the urea water flowing in the urea water supply piping from being frozen in the winter season (see Patent Document 1).